Kelly put it great!
To some extent you can go by the % drop. You are generally looking for something in the 60-65% drop range, though ECID applies. Bigger than that is usually too steep, shallower than that and you likely won't get a very good nadir. On a lower PS there is typically more upward pressure on the BGs so to speak, so from a higher PS sometimes they will sort of free fall down to the nadir, vs. on a lower PS the insulin is countering that upward pressure rather than really dropping the #s down all that far (varies by the zoom rate they are having). That's kind of made up, but that's how I picture it, if it makes any sense.
I would lower slightly on a lower PS as you collect data, but not by a lot. Shoot the most you feel safe shooting and then get some spot tests in, and then if it isn't enough you will know you can increase for next time you see a similar PS.